The present invention relates to an optical reproducing apparatus which performs a three beam type tracking error detection and particularly to an optical reproducing apparatus suitable for the simplification of an optical construction thereof.
Heretofore, as a three beam type tracking error detecting means there has been known, for example, such a construction as shown in U.S. Pat. No. 4,592,038 in which a diffraction grating having an equally spaced, rectilinear groove pattern is disposed in an optical path between a semiconductor laser beam source and a beam splitter perpendicularly to the optical axis. According to such conventional arrangement, it is possible to effect the detection of tracking error using a simple optical system. However, there has been the problem that two special optical components are needed, one being a beam splitter for the separation of light beam incident on an optical information recording medium (hereinafter referred to as the "optical disc") and light beam reflected from the optical disc, and the other being a diffraction grating for separating incident light into three beams.
In view of the above problem the present inventors have made studies about a new optical element as an integration of both such beam splitter and diffraction grating to reduce the number of components used. According to this new optical element, a beam splitter film is formed on the groove portion of a reflection type diffraction grating, whereby not only light beam incident on the optical element is separated into reflected light beam and transmitted light beam, but also the reflected light beam is separated into zero order light beam and .+-.1st order light beams.
But in the use of such an integrated optical element of a beam splitter and a diffraction grating, it became clear that there arose the following problem. Since its grooved side is disposed inclinedly relative to the optical axis, .+-.1st order diffracted light beams increase in wave front aberration in such a conventional diffraction grating as has an equally spaced, rectilinear groove pattern, resulting in that the spots of the .+-.1st order light beams applied onto an optical disc are markedly deteriorated in their focused condition.